1. Field of the Invention
This invention relates to an auto-focus apparatus of the type in which an auto-focus measurement value is obtained from contrast components of a video signal.
2. Description of the Prior Art
In a conventional auto-focus apparatus that is part of an image processing system, an auto-focus operation is carried out by adjusting a focusing lens so that an auto-focus measurement value obtained from contrast components of a video signal is maximized. This is based on the concept that proper focus is achieved when the high frequency components of the video signal are at a maximum. More particularly, as shown in FIG. 1, the direction and speed of adjustment are controlled on the basis of the sign and magnitude of a change .DELTA.y in the auto-focus measurement value which results when the focusing lens is adjusted by a very small distance .DELTA.x.
FIG. 2 shows a conventional circuit for generating an auto-focus measurement value. As shown in FIG. 2, a digital video signal Y is received at an input terminal 1 and is band-limited by a high-pass filter (HPF) 2. The band-limited signal output from HPF 2 is converted into an absolute value signal representative of contrast components of the video signal by an absolute value (ABS) circuit 3. The signal output from the ABS circuit 3 is supplied to a detecting unit 4 which generates an auto-focus measurement value EST on the basis of the absolute value signal according to one of various conventional approaches which will be described below. The resulting auto-focus measurement value EST is then output from an output terminal 5.
In describing the ways in which the auto-focus measurement value EST may be obtained according to the above-mentioned conventional approaches, S.sub.1,1 - - - S.sub.1,k2 ; S.sub.2,1 - - - S.sub.2,k2 ; . . . ; S.sub.k1,1 - - - S.sub.k1,k2 will represent absolute value (contrast component) signals output from ABS circuit 3 in response to a frame (hereinafter referred to as the "range finder frame") that is part of a field or frame of the video signal from which the auto-focus measurement value EST is to be obtained.
A conventional approach known as an "integration system" will first be described. According to this system, as shown in the following equation (1), all of the signals S.sub.1,1 through S.sub.k1,k2 are added in the detecting unit 4 and the resulting sum is provided as the auto-focus measurement value EST (see FIG. 3A). ##EQU1##
Next a so-called horizontal line (H line) peak hold system will be described. According to this system, as shown in the following equation (2), there is generated a maximum value of each line of signals (S.sub.1,1 - - - S.sub.1,k2), (S.sub.2,1 - - - S.sub.2,k2), . . . , (S.sub.k1,1 - - - S.sub.k1,k2). The line maximum values are summed and the result is provided as the auto-focus measurement value EST (see FIG. 3B). ##EQU2##
A third conventional system, known as the vertical peak (V peak) hold system will next be described. According to this system, as shown in the following equation (3), the maximum of all of the signals S.sub.1,1 - - - S.sub.k1,k2 is detected by detecting unit 4 and provided as the auto-focus measurement value EST (see FIG. 3C). ##EQU3##
All of these conventional systems suffer from disadvantages. An auto-focus measurement value obtained according to the integration or H line peak hold system is not greatly affected by noise but tends to be affected by a moving object within the scene represented by the video signal. Conversely, an auto-focus measurement value obtained according to the V peak hold system is not greatly affected by moving objects within the scene of the video signal, but tends to be affected by noise.
Turning to another aspect of conventional approaches for generating an auto-focus measurement value, reference is again made to the curve shown in FIG. 1, which represents changes in the auto-focus measurement value in response to changes in position of the focusing lens. In order to perform a satisfactory auto-focus operation, the base of this curve must be inclined. For that purpose, HPF 2 (FIG. 2) is required to have a low cut-off frequency so that its output signal contains as many components as possible other than DC.
However, the lower the cut-off frequency of HPF 2, the longer the time period during which a signal change at a certain point in time continues to affect the output of HPF 2. FIGS. 4A and 4B respectively show the step response and the frequency characteristic of embodiments of HPF 2 depending on variations in its transfer characteristic (1-D)/(1-kD). It will be seen that as the parameter k is increased, the cut-off frequency is decreased, but the period of response to a signal step increases.
Other problems with conventional auto-focus systems arise when an edge of an object is present near the left side of the range finder frame, or when the leading edge after a horizontal blanking interval (i.e. the black level) affects the output of HPF 2. FIG. 5A shows the input video signal Y and FIG. 5B shows the corresponding output signal from HPF 2. As seen from FIGS. 5A and 5B, the output signal of the HPF 2 during the period corresponding to the range finder frame is affected by the leading edge EDGE which follows the horizontal blanking period.
Another disadvantage of conventional auto-focus systems will be explained with reference to the flow chart shown in FIG. 6, which illustrates a process according to the prior art for determining whether an auto-focus operation should be performed.
It should be understood that the automatic focus operations described herein are conducted with respect to real-time moving images, not still pictures. Accordingly, when a change occurs in the auto-focus measurement value, it is necessary to distinguish between two different cases: (1) An object within the scene has moved but the scene as a whole is unchanged; and (2) the entire scene has changed. In the second case, a complete auto focus operation must be performed; in the first case only a fine adjustment should be made, for otherwise the picture would become unstable.
At the beginning of the routine shown in FIG. 6, it is determined, at step 31, whether or not the auto-focus measurement value has changed from the value for the preceding field. If so, then the routine proceeds to step 32 in which a field count t is set to an initial value t0 (for example, t0=20).
Following step 32 is step 33, at which it is determined whether the auto-focus measurement value for the next field has changed. If not, the routine returns to step 31. Otherwise, following step 33 is step 34, at which the field counter t is decremented. After step 34 is step 35, at which it is determined whether the field counter t has been decremented to 0. If not, then the routine returns to step 33. Otherwise, i.e. if the auto-focus measurement value has changed for a consecutive number of fields equal to t0, then it is determined that an auto-focus operation should be performed (step 36).
It is a disadvantage of this prior art decision making process that the start of the auto-focus operation does not occur for a relatively long time, e.g., 20 fields, after a change of scene. However, if the number of fields t0 is reduced, then it often occurs that motion of an object within the scene is mistaken for a change of scene, resulting in such problems as an unstable picture.